JTP - Just sent you a PM....again, sorry for the delay in getting back to you. Been over a month since I logged in to the forum, sadly.
OP - I haven't used the boxster scissor jack, but I absolutely hate scissor jacks. Feel they are dangerous.
If you simply need a bit more height to get your jack underneath, I recommend driving up on 4 blocks of 2x4s for an extra 1.5" of clearance.
Also, I have this jack:
http://www.harborfreight.com/rapid-pump-4000-lb-capacity-lightweight-low-profile-aluminum-racing-jack-92782.html
Despite a few of the reviews, I can tell you that it will get just about anywhere under the boxster and lift it effortlessly. I just swapped on my blizzaks with this + a 2-ton jack stand and, aside from the PITA that are wheel studs (instead of lugnuts), it was a piece of cake